Lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. Consequently, in recent years, studies have been made to improve battery members such as electrodes for the purpose of achieving even higher lithium ion secondary battery performance.
A positive electrode for a secondary battery such as a lithium ion secondary battery generally includes a current collector and a positive electrode mixed material layer formed on the current collector. The positive electrode mixed material layer is formed, for example, by applying, onto the current collector, a slurry in which a positive electrode active material, a conductive material, a binding material, and so forth are dissolved or dispersed in water, an organic solvent, or the like, and drying the applied slurry. In recent years, there have been attempts to improve positive electrode active materials and binding materials in order to further improve dispersion stability of slurries used to form secondary batteries and electrical characteristics of secondary batteries.
For example, PTL 1 proposes improving cycle characteristics, thermal stability, and so forth of a secondary battery by using, as a positive electrode active material, a powder of lithium cobalt oxide particles that contain Zr in a specific content ratio and that are coated with a Zr compound represented by a specific chemical formula. Specifically, PTL 1 proposes improving cycle characteristics, thermal stability, and so forth of a secondary battery by using a positive electrode that is formed using a slurry containing the aforementioned positive electrode active material, a conductive material such as acetylene black, and a binding material such as polyvinylidene fluoride.
In another example, PTL 2 proposes improving stability of a slurry for a positive electrode of a secondary battery and also improving cycle characteristics of a secondary battery by using, as a binding material, a polymer that includes a nitrile group-containing monomer unit, a (meth)acrylic acid ester monomer unit, a hydrophilic group-containing monomer unit, and a straight chain alkylene structural unit having a carbon number of at least 4, and that exhibits a degree of swelling of from 100% to 500% with respect to a specific electrolysis solution.